Weldable high strength structural steel not embrittled by stress-relieving annealings



United States Patent 3,494,765 WELDABLE HIGH STRENGTH STRUCTURAL STEEL NOT EMBRITTLED BY STRESS- RELIEVING ANNEALINGS Hisashi Gondo, Tadashi Nishi, and Mizuo Sakakihara, Kitakyushu, Fukuoka Prefecture, Japan, assignors to Yawata Iron & Steel Co., Ltd., Tokyo, Japan No Drawing. Filed July 19, 1967, Ser. No. 654,345 Claims priority, application Japan, July 21, 1966, 41/47,849 Int. Cl. C22c 39/50, 39/54, 39/44 US. Cl. 75123 1 Claim ABSTRACT OF THE DISCLOSURE A weldable high strength structural steel, the mechanical properties of which are not deteriorated by subjecting it to repeated stress-relieving annealings for a long time at temperature below the Ac transformation point, characterized by the composition containing less than 0.15% carbon, less than 0.5% silicon, 0.5 to 2.0% manganese, less than 2.00% nickel, 0.20 to 0.50% molybdenum, 0.03 to 0.20% niobium and, if necessary, less than 0.2% in total of at least one element selected from the group consisting of vanadium and zirconium and the balance being iron and unavoidable impurities.

This invention relates to a weldable structural steel, more particularly, a weldable high strength structural steel, the mechanical properties of which are not deteriorated, even when it is subjected'to stress-relieving annealings carried out repeatedly over a long period of time at a temperature below the Ac transformation point after it has been weld-ed. 7

It is well known that when known Mn-Mo series low alloy steels among conventional structural steels are subjected to repeated stress-relieving annealings in order to relieve residual stress caused by the welding thereof a deterioration is caused in mechanical properties, particularly in impact strength, of the mother metals. Therefore, special considerations are required for the designing or the application of these conventional low alloy steels or special controls of heat-treatment are needed.

The object of the present invention is to provide a weldable low-alloy structural steel, which possesses a high tensile strength and at the same time shows no deterioration in mechanical properties even after subjected to repeated stress-relieving annealings.

Other objects of the present invention will be readily understood from the description set forth hereinafter.

That is, the steel of the present invention has the following chemical composition: less than 0.15% carbon (by weight percent), less than 0.50% (preferably less than 0.35%) silicon, 0.50 to 2.00% manganese, less than 2.00% nickel, 0.20 to 0.50% molybdenum, 0.03 to 0.20% niobium and, if need be, less than 0.20% in total at least one element selected from the group consisting of vanadium and zirconium and the balance being iron and unavoidable impurities.

A marked embrittlement, which usually appears in Mn-Mo series alloy steels when they are subjected to stress-relieving annealings, means a deterioration in mechanical properties ascribed to metallographic changes in microstructure taking place during a heating carried out at high temperature for a long time and a very slow cooling process following it which is again mainly attributed to changes in concentration of manganese and molybdenum enriched in cementite and concentration gradients in its boundary regions and further to changes in the composition and distribution form of Mo-series carbide due to a long-lasting heating.

It was discovered by the inventors of the present invention that by adding niobium to the said Mn-Mo steels and subjecting the resultant steels to a heat-treatment under appropriate conditions an activating energy of carbon may be reduced, thereby reducing the rate of enrichment of manganese and molybdenum in cementite and exercising an eflFect on delaying the formation of carbides. Consequently, even subjecting the steel containing niobium to repeated stress-relieving annealings, there is seen no deterioration in mechanical properties, particularly ductility of the mother steel. Rather, by the addition of niobium ferrite grains are refined, whereby not only the ductility of the steel can be improved, but also the tensile strength can be improved. Moreover, as the formation of Mo series carbides can be prevented the effects of adding molybdenum and manganese on improving the strength can fully be realized.

It was also found that a further addition of either zirconium or vanadium or both of them has the effect of improving mechanical properties of conventional alloy steels subjected to stress-relieving annealings.

By the tests, in which steel sheets of over millimeters in thickness, for one example, were subjected to stress-relieving annealings several times at a temperature of 625 C., there has been shown that in the case of alloy steels containing no niobium, zirconium and vanadium the V-notch Charpy impact test values at 12 C. below zero were reduced from 10-20 kgm./cm. of the mother steels to below 5 kgm./ 0111. while in the case of adding these elements to the said steel sheets in proper amounts the embrittlement due to stress-relieving annealings was prevented and the strength at a high temperature was improved.

The reasons why the chemical composition of the alloy steel according to the present invention is limited to the range referred to hereinafter will be explained.

In order to obtain good weldability and improved mechanical properties of an alloy steel even after it has been subjected to repeated stress-relieving annealings the carbon content of the steel must be low. Originally, carbon is added to increase such a tensile strength of the alloy steel as aimed by the present invention, but it is desirable to be less than 0.12%. Particularly, if the added amount of carbon exceeds 0.15%, the impact value is reduced and the mechanical properties are rapidly deteriorated as the number of times of stress-relieving annealmgs increases.

It is desirable that the silicon content of the alloy steel be minimized from the viewpoint of preventing deterioration in mechanical properties and graphitization caused by stress-relieving annealings. However, silicon may be added in an amount up to 0.5%, preferably up to 0.35%, in accordance with a necessity of improving mechanical properties of the mother steel attended with an increase in the thickness of the steel sheet. However, the addition of silicon of more than 0.5% is not desirable, because it promotes the graphitization and remarkably causes the embrittlement due to the stress-relieving annealings.

Manganese has little effect on deteriorating material qualities due to the stress-relieving annealings. An addition of manganese in an amount more than 0.5% is, however, required in order to improve mechanical properties of the mother steel. But, an addition of manganese in an amount of more than 2.00% is not desirable, because it tends to reduce the weldability and accelerate the embrittlement due to stress-relieving annealings.

Nickel is added in an amount of less than 2.00% for the purpose of improving mechanical properties of the mother steel. Moreover, nickel has an eliect of improving the mechanical properties of the steel subjected to stressrelieving annealings. However, an addition of nickel in an amount of more than 2.00% increases the production cost, whereby the advantage to be achieved by the addition of nickel will be lost. On the other hand, with an addition thereof in an amount of less than 0.15% the effect of adding nickel as above mentioned can not be realized.

The addition of molybdenum should be so small as possible from the viewpoint of substantially checking the deterioration in the impact characteristic of the steel caused by the stress-relieving annealings. However, the addition of molybdenum is, on the other hand, required for preventing the deterioration in the strength of the mother steel and also of the steel which has been sub- 4 ing the embrittlement cause by the stress-relieving annealings. In order to obtain this purpose the addition of these elements in an amount of more than 0.01%, preferably more than 0.03%, is necessary, but the addition of more than 0.2% is not desirable, because the impact property and weldability of the alloy steel will thereby be impaired and also the production cost will be higher.

In the following examples of the present invention will be explained. It is, of course, to be understood that the invention is not limited to such examples and 'variations may be made therein without departing from the spirit an scope of the invention.

TABLE 1.-CHEMICAL COMPOSITIONS OF THE STEELS OF THE PRESENT INVENTION AND REFERENCE STEEL Reference Steels of the present invention steel Sample Q R T S X Chemical composition (wt. percent):

Carbon O. 10 0. 06 0. 08 Silicon 0. 30 0. 28 0. 25 Phosphorus 0. 015 0. 017 O. 014 U1 0. 013 0. 104 0. 011 Manganes 1. 42 1. 38 1. 65 Ni 0. 85 1. 1. 45 Molybdenum 0. 26 0. 47 0. 25 Vanadium 0. 0. 05 0. 03 Niobium 0. 04 0. 05 0. 09 Zh-mm'um 0 ()4 Iron Balance Balance Balance Balance Balance TABLE 2.MECHANICAL PROPERTIES OF MOTHER MATERIALS OF THE STEELS HAVING CHEMICAL COMPOSITIONS ACCORDING TO TABLE 1 AND THOSE SUBJECTED TO STRESS-RELIEVING ANNEALINGS FIVE TIMES REPEAIEDLY Heat-treatment Mother material: 930 (1X4 hr. (air cooling) plus 650 0.

X4 hr. (air cooling) Stress-relieving annealing 5 times! Mother material plus 625 (1X4 hr. (cooling in furnace 5 times) Y.P., T.S., vTrS, vTr 30, vE-12, Y.P., T:S., vTrS, vTr 30, vE -12,

Mechanlcal properties Kg./mm. KgJmm. 0. C. KgmJcm. KgJmm. KgJmm. 0. C. KgmJcrn.

51. s 66. 7 -31 1s. 5 52. 1 e5. 8 I53 -33 19. 5

REmnKs.Y.P. is yield point. T.S. is tensile strength. v'lrS is V- noteh shear fracture transition temperature, that is, a temperature at which qt the area of the total shear fracture is a ductility fracture (v is an abbreviation of V-notch, Tr is that of transition temperature and S jected to the stress-relieving annealings, at a normal temperature and also a high temperature. In balancing the above mentioned two requirements molybdenum is necessary to be added in an amount of more than 0.20% in order to effect the improvement of the strength. Particularly, a remarkable efiect can be obtained with an addition of molybdenum in an amount of more than 0.33%. However, the addition of more than 0.50% is not desirable, because in this case, the said eiiect will be oifset by the deterioration in material qualities caused by the stressrelieving annealings.

As above mentioned, niobium was discovered by the inventors of the present invention to be an element, which has -a distinguished effect on preventing the embrittlement caused by stress-relieving annealings. Niobium must be, however, added in a range from 0.03 to 0.2%. If less than 0.03%, the egect cannot be produced, but if more than 0.5%, the steel becomes too hard and the effect of preventing the embrittlement due to the stress-relieving annealings will be already saturated, which makes the addition of niobium meaningless.

Vanadium or zirconium orboth of them are added in the range of.0.01 to 0.20% in total as the occasion demands for .improving the strength of alloy steel either at a normal temperature or a high temperature. Vanadium and zirconium are useful alike niobium also for preventisthat of shear fracture) vTr 30 is V-notch 30 It-.lb transition temperature at which an energy of 30 [t.-lb. is given. V15 12 is V-notch Charpy impact value at 12 C The samples shown in Table 1 are steel sheets of millimeters thick produced through the steps of ingot casting, slabbing and hot-rolling according to a conventional steel making process. Table 2 shows a comparison with respect to variations in mechanical properties caused by difierently subjecting the samples produced under the same conditions of production and the same history of heat-treatment to the stress-relieving annealings. It is evidently seen from the Table 2 that the samples Q, R, T and S of the present invention are high in strength and markedly manifest the efi'ect of preventing the embrittlement to be caused by repeated stress-relieving annealings.

Having thus described the invention, what is claimed is:

1. A weldable high tensile strength structural steel which isnot embrittled even when subjected to repeated stress-relieving annealings, comprising less than 0.15% (by weight) carbon, less than 0.5% silicon, 0.50 to 2.00% manganese, less than 2.00% nickel, 0.20 to 0.50% molybdenum, 0.03 to 0.20% niobiu m and 0.03 to 0.20% in total of at least one element selected from the group consisting of vanadium and zirconium and the balance being Fe and unavoidable impurities. u

(References on following p g References Cited UNITED STATES PATENTS OTHER REFERENCES Grange et a1.: B, Ca, Cb and Zr in Iron and Steel, John Wiley & Sons, Inc., N.Y., 1957, relied on pages 153, 159-167, 171, 172, 462, 463 and 467. Becket et a1. 75123 Becket er a], 5 12 X 5 CHARLES N. LOVELL, Primary Examiner Shimmin et a1. 75123 CL Hodge 75123 14 3 

